1. Field of the Invention
The present invention relates to a mobile communication system, base station, mobile station and mobile communication control method.
2. Description of the Related Art
A threshold value associated with DHO (diversity handover) in conventional mobile communication is defined in a mobile station as a value common to all base stations.
FIG. 1 is a diagram illustrating a conventional method for deciding a DHO addition candidate (a candidate of a base station with which a mobile station will start new communication). Let us assume here that a base station always transmit a reference signal (such as a perch signal, for example) at fixed transmission power in addition to a signal for communicating with a mobile station. In FIG. 1, the vertical axis represents received powers of perch signals by a mobile station. A mobile station always measures received powers of perch signals from base stations, and decides a DHO addition threshold value as follows. It selects the highest received power among received powers of perch signals from multiple base stations (or a single base station) which are currently in communication with the mobile station as a reference value, and determines the received power lower than the reference value by X0 dB as the DHO addition threshold value. When a base station in non-communication (a base station which is currently not in communication with the mobile station) provides perch received power (received power of perch signal) equal to or greater than the DHO addition threshold value, the base station is selected as a DHO addition candidate. As for the example of FIG. 1, since the perch received power of the base station B is greater than the DHO addition threshold value, the base station B is selected as a DHO addition candidate. On the other hand, since the perch received power of the base station C is less than the addition threshold value, the base station C is not selected as a DHO addition candidate.
FIG. 2 is a diagram illustrating a conventional method for deciding a DHO deletion candidate (a candidate of a base station with which a mobile station will stop current communication). A mobile station always measures received powers of perch signals from base stations, and decides a DHO deletion threshold value as follows. It selects the highest received power (VA in the example of FIG. 2) among received powers of perch signals from multiple base stations (or a single base station) which are currently in communication with the mobile station as a reference value, and determines the received power lower than the reference value by Y0 dB as the DHO deletion threshold value. When a base station in communication provides perch received power equal to or less than the DHO deletion threshold value, the base station is selected as a DHO deletion candidate. As for the example of FIG. 2, since the perch received power VC of the base station C is less than the deletion threshold value, the base station C is selected as a DHO deletion candidate. On the other hand, since the perch received power VB of the base station B is greater than the deletion threshold value, the base station B is not selected as a DHO deletion candidate.
FIG. 3 is a diagram illustrating an example of deciding a DHO addition candidate in conventional control. First, let us assume that a mobile station 10 is present in the (service) area of a base station 20, and is communicating only with the base station 20. The mobile station 10 calculates the DHO addition threshold value using the value X0 dB sent from the base station 20. The mobile station 10 ignores another value X1 dB (<X0 dB) a base station 30 may send for the mobile station to calculate the DHO addition threshold value. Here, assume that the mobile station 10 is moving toward the base station 30. When the mobile station 10 approaches the base station 30 to a certain distance, the perch received power of the base station 30 exceeds the DHO addition threshold value. In FIG. 3, the reference numeral 32 designates an area where the perch received power of the base station 30 exceeds the DHO addition threshold value, and 31 designates the radius of the area 32. Thus, when the mobile station 10 enters the area 32, the base station 30 becomes a DHO addition candidate. Incidentally, if the base station 20 sends the value X1 dB for the mobile station to calculate the DHO addition threshold value, and the mobile station 10 calculates the DHO addition threshold value using the value, the base station 30 becomes a DHO addition candidate when the mobile station 10 enters an area 34.
FIG. 4 is another diagram illustrating an example of deciding a DHO addition candidate in conventional control. First, let us assume that the mobile station 10 is present in the (service) area of the base station 30, and is communicating only with the base station 30. The mobile station 10 calculates the DHO addition threshold value using the value X1 dB sent from the base station 30. The mobile station 10 ignores another value X0 dB the base station 20 may send for the mobile station to calculate the DHO addition threshold value. Here, assume that the mobile station 10 is moving toward the base station 20. When the mobile station 10 approaches the base station 20 to a certain distance, the perch received power of the base station 20 exceeds the DHO addition threshold value. In FIG. 4, the reference numeral 24 designates an area where the perch received power of the base station 20 exceeds the DHO addition threshold value, and 23 designates the radius of the area 24. Thus, when the mobile station 10 enters the area 24, the base station 20 becomes a DHO addition candidate. Incidentally, if the base station 30 sends the value X0 dB for the mobile station to calculate the DHO addition threshold value, and the mobile station 10 calculates the DHO addition threshold value using the value, the base station 20 becomes a DHO addition candidate when the mobile station 10 enters an area 22.
As for decision of a DHO deletion candidate, it is analogous to that of a DHO addition candidate.
As described above, threshold values associated with DHO in conventional mobile communication are defined in a mobile station such that they are common to all the base stations. They are not varied in real time in response to the operation of each base station. Therefore, it is impossible for a base station to overcome a shortage of the radio channel capacity even if this is possible by varying DHO addition threshold value to reduce the number of DHO branch connections of the base station when the radio channel capacity is not enough. Likewise, it is impossible for a base station to overcome a shortage of the radio channel capacity even if this is possible by varying DHO deletion threshold value to release DHO branch or branches connected to the base station.